Chemistry Unit 2

Unit 2.2: Rates of reaction

Calculating rates

Chemical reactions occur at a definite rate determined by the reaction conditions. The rate of reaction may be increased by:

1. Increasing temperature
2. Increasing concentration of reactants
3. Increasing surface area

Many reactions are subject to kinetic control.

For example:

C_(s) + O_2(g) → CO_2(g) is an energetically favourable reaction but coke does not burn spontaneously in air at room temperature. This is because the reaction rate is too slow. This reaction is controlled by kinetic factors.

To follow the rate of a reaction, one must either measure the decrease in concentration of a reactant or the increase in concentration of a product with time.

Some techniques for doing this are:

1. Measure the volumes of gases evolved (gas syringe).
2. Volumetric analysis - samples are removed at regular intervals, the reaction stopped by cooling, and mixture analysed by titration.
3. Measuring changes in pressure (for gas reactions)
4. Colorimetry may be used if one of the constituents is coloured. The colorimeter follows the change in intensity of colour.
5. A conductivity meter may be used if there is a change in conductivity during the reaction i.e. if the number of ions present is changing. A pH meter is a special type of meter which will follow changes in H⁺.

Typical results

Consider the reaction:

Mg_(s) + 2HCl_(aq) → MgCl_2(aq) + H2_(g)

This may be followed by measuring the volume of H₂ collected in the gas syringe at intervals of time:

The rate of reaction at a particular time is given by the gradient of the tangent.

At time t: rate = change of volume / time or a / b

Rates are usually expressed by the calculus notation.

The units are cm^-3 s^-1 or mol^-1 s^-1

The rate of a reaction is fastest at t = 0 and decreases steadily as the reactants are used up.

Hence, the gradient of the graph decreases with time.

When examining the effect of changing conditions (e.g. concentration) on the rate of reaction, we usually take the initial gradient, at t = 0 as a measure of rate. We can then compare the initial rates of reaction under the different conditions and determine any effect.

The rate of reaction– the change in concentration of a reactant or product per…